Deep ocean circulation during the early Eocene: a model-data comparison

Abstract

In a business as usual, high emissions scenario, the IPCC projects CO2 levels to exceed 1000 ppm by the end of the century. The last time atmospheric CO2 surpassed this threshold was ∼50 million years ago during the early Eocene climatic optimum (EECO). The EECO presents the warmest sustained temperatures of the Cenozoic, with global mean surface temperatures (GMSTs) ∼14◦C higher than the preindustrial. As such the EECO is an increasingly utilised time period for ground truthing climate models with real world data, to better understand climate dynamics during a period of extreme warmth. This thesis aims to provide constraints on deep ocean circulation during the EECO from a combined model-data perspective. Neodymium isotope analyses of a total of 16 global DSDP, ODP and IODP cores are used to determine regions of deep water formation, export and water mass mixing, firstly at a low temporal resolution and then at orbital resolution. A total of eight model simulations are carried out using a newly tuned version of the DeepMIP model HadCM3BL, varying both CO2 level and orbital configuration. These results are utilised in model-data comparisons of previously published sea surface temperature proxy data and the newly informed Nd isotope inferred overturning structure. Global Nd data suggest a circulatory regime driven by the southern hemisphere, with multiple, discrete regions of deep water formation around Antarctica. This is consistent with model outputs with a modern orbital configuration at both CO2 levels. Neodymium data furthermore indicate the earliest onset of transient northern component water (NCW) formation off Baffin Bay, and model outputs suggest that these intermittent pulses could be driven by transitions to lower CO2 levels or minimum seasonality orbits, both associated with lower GMST. Although models indicate a strong orbital control on ocean circulation, this is not evident in high resolution Nd data.